Trains are widely used to transport people and freight. Freight trains in particular may include 150 or more railcars and extend over a mile or more. Control is required for operating the railway braking system to ensure proper braking.
Compressed air is commonly used as a power source in railway braking systems because it is essentially inert and there are no associated environmental hazards. The use of electrical control devices in conjunction with such pneumatic equipment significantly improves the ability to control the compressed air and therefore improves the effectiveness of the equipment.
A typical railway braking system includes a reservoir of compressed air to supply a relay valve which controls a pneumatic brake cylinder. The relay valve, in turn, is controlled by one or more electromagnetic solenoid valves which can vent the relay valve to atmosphere or connect it to the supply reservoir.
The current state-of-the-art is to use wireless technology to control various devices within railcars and locomotives. U.S. Pat. Nos. 4,582,280 and 4,553,723 to Nichols et al., and assigned to the assignee of the present application, are seminal patents directed to a radio communication based train control system. GE Harris Railway Electronics, L.L.C. offers a radio based control system under the designation LOCOTROL.RTM. which provides coordinated distributed power and air brake control from the lead locomotive as described in the above referenced patents. Furthermore, GE Harris offers the EPx.TM. Direct Braking System which is an Electronically Controlled Pneumatic (ECP) brake system that uses wireless communications technology to communicate braking commands to each railcar.
Railway pneumatic braking equipment requires relatively large orifices to permit the necessary air flow while allowing reasonably low operating pressures. For pneumatic applications which require electrical flow control devices to have a coefficient of flow greater than about 1.0, the current state-of-the-art is to use one of two types of electrical control devices. The first type is a directly operated electrically controlled pneumatic device. A significant consequence of using such a device is the extremely large amount of electrical power required to directly seal such a large orifice. This is because the force of the air pressure is proportional to the square of the surface area of the orifice.
The second type of electrical control device is a pilot valve or device. A pilot valve controls a small flow of air and uses the air pressure to develop large enough forces to operate secondary pneumatic control devices, such as the relay valve. The relay valve then controls the pneumatic brake cylinder, for example. This has the advantage of providing electrical control at a reasonable cost in return for slightly slower operation.
Conventional electrical pilot control devices are usually electromagnetic solenoids which require multiple Watts of power and cannot be considered low power. For most larger air-flow applications, such as industrial plants, a power draw of multiple Watts is insignificant. However, railcars do not typically have on-board electrical power generators. The cost and logistics of generating the amount of electrical power required for conventional electrical control devices for a railway car braking system may be prohibitive.
A significant contributor to power consumption is the mitigation of an important failure mode of a freight train. This failure mode occurs when the brakes are locked in the applied position without the ability to detect this condition. This condition is commonly referred to as dragging brakes. A train car with severely dragging brakes may go unnoticed until the wheel overheats, and shatters. Electrically controlled pneumatic brake controllers can detect and warn of this situation as long as they are operating and powered. However, if the controller becomes disabled, the system on that car must release the brake cylinder pressure to avoid damage.
One potential failsafe method is to have the electrically controlled pilot valve only hold pressure when electrical power is applied. In this way, if the control system becomes disabled, the natural rest state of the brake system is the desired brake release state. This requires that electrical power must be supplied whenever brakes are applied, which can be for multiple hours. Due to the limited electrical power available in a freight car, the pilot valve must have extremely low electrical power draw.